1. Technical Field
This application generally relates to digital communication systems and methods, and particularly relates to equalizer circuits.
2. Description of the Related Art
The Digital Visual Interface (DVI) Specification, Revision 1.0, dated Apr. 2, 1999, and published by Digital Display Working Group, provides for a high speed digital connection for visual data types that is display technology independent. A DVI interface is typically focused on providing a connection between a computer and the computer display device. A DVI system uses a transition minimized differential signal (TMDS) for a base electrical connection, in which 8 bits of data are encoded into a 10-bit, transition minimized DC balanced character.
DVI accommodates several different serial signal rates, the highest of which is a signal rate of 1650 Mb/s. This signal rate corresponds to a data rate of 825 MHz. The DVI data may be transmitted over a video bus in a computer device, such as in laptop computer, or may be transmitted over a cable that is external to a computer device, such as a video cable used to connect a remote monitor to a computer. Typically, cables over short distances and low frequencies can be considered ideal channels having minimal loss and a bandwidth much greater than the input signal. The ideal cable with infinite bandwidth produces no dispersion of the input data.
Real cables, however, have a loss characteristic that is a function of the data frequency and the cable length. Thus, the longer the cable length, the greater the loss characteristic. In practical applications, the attenuation of the high frequency components of the DVI data signal at 1650 MHz typically limits DVI cable lengths to about 5 meters.
Equalizers may be used to restore the integrity of the DVI data so that the cable length between the source and the destination does not reduce the system performance. Many equalizers comprise a differential pair having an automatic gain control (AGC) feedback block between the output of the differential pair and the inputs of the differential pair. Additionally, many of these differential pairs utilize inductors, which demand a relatively large amount of semiconductor area and are susceptible to noise.
The DVI specification also supports the VESA Display Data Channel (DDC), which enables the computer display, the computer, and a graphics adapter to communicate and automatically configure the system to support different features available in the computer display. The DDC link is typically a lower bandwidth signal, e.g., 400 kHz, and thus may be transmitted over a longer cable length than the DVI data signal. However, the DDC cable is typically not terminated in an impedance match, and thus reflections in the DDC cable may degrade the DDC signal as the DDC cable length increases. Additionally, the bandwidth of the DDC signal is limited by the amount of pull-up current injected into the DDC cable during a transition of the data signal from a low voltage level to a high voltage level.